The present invention relates generally to borescopes or endoscopes for viewing inaccessible areas which are not directly viewable by eye, and is particularly concerned with endoscope instruments as used in surgery and diagnostics to allow doctors to view regions inside the body through a relatively small incision.
Typical surgical endoscopes have an optical system combined with an elongate optical pathway or channel, which may be rigid or flexible, for transmitting an image formed at one end of the channel to the opposite end of the channel. Simple endoscopes have only one optical channel and create only a two-dimensional or monoscopic view of the region under inspection. This will lack depth perception, making it difficult to perform an accurate inspection or surgery. Thus, three-dimensional or stereoscopic endoscopes have been developed for creating a three-dimensional view of the object or region under inspection. These endoscopes are provided with a pair of optical paths or image guides for transmitting right- and left-hand images of the object to a stereoscopic viewer. The stereoscopic viewer has microscope-like eyepieces through which the surgeon views the respective images. The eyepieces are arranged so that the surgeon's eyes provide the necessary convergence to combine the images into a stereoscopic view. Stereoscopic endoscope systems of this type are described, for example, in U.S. Pat. No. 3,520,587 of Tasaki et al. and U.S. Pat. No. 4,651,201 of Schoolman.
One major disadvantage of the known stereoscopic endoscope systems is that the two parallel optical systems used in such arrangements cannot converge the images and will provide two separate images or video pictures where a video viewing system is used, the images having a large relative offset to the image center. Although the human brain can converge and "fuse" two separate views if the separation between the images is not too great, this is not easy or comfortable to achieve in practice. In typical stereo-microscopes, the problem is solved by using two converging optical systems.
Convergence of right eye and left eye images of an object is done in normal stereopsis by converging the optical axes with the eyes or optical/mechanical means to accomplish convergence of the right and left images so that the brain receives and perceives the images as sufficiently close together for the brain to combine (fusion) the images as a single three-dimensional image. In normal unaided vision, this is accomplished in Panum's Fusional Area and by optical/mechanical means the same result can be obtained at much shorter focal distances than the eyes can converge. The stereomicroscope is an example of such an optical/mechanical device. However, this is not a practical solution in endoscopes where the necessary convergence at very short focal lengths is compounded by the need to keep the overall diameter of the system as small as possible so that the endoscope tube can be inserted through a single minimum size surgical incision, minimizing invasive procedures. The optics of the endoscope must therefore be small enough to be enclosed in a single relatively small diameter tube.